Marine radio was a monopoly because of the Marconi Patents. In 1919 Scot-Taggart applied for a patent for a new type of oscillator - a negative resistance device the Negatron. This featured a pair of anode with a filament at the centre and a grid between one anode and the filament.
Detailed drawings show a ladder grid and a side contact for the second anode. Research by Wolfgang Holtmann with a replica Negatron was written-up and can be found as a PDF here.
This valve was made by Mullard in around 1926. The other name on the glass is RCC Polar. The Radio Communications Co used the trade name Polar. Clearly they wanted to be part of the marine radio business. RCC were backers of Mullard and helped him found the Mullard business. Scot-Taggart worked for RCC during this period. So Mullard making the Polar Negatron is not unreasonable.
The Negatron was used for only a short period and in professional communications, consequently very few examples exist. This example has just a four pin B4 base - no side contact. The second anode may connect to the base shell or the grid my be connected to one side of the filament.
The G on the base cap is by pin 2 as would be expected. The a the other side is on pin 1. The side support is for the filament tension spring.
The pins are not of equal length - pin 3 is longer and thus would make contact first.
The base pins are simple split-pins. The Mullard name is embossed on the base.
The etched triangle has RCC and Polar. The number below is 2382.
The other side of the envelope has the Mullard logo and Made in Britain. The wording above the logo is Registered Trade Mark.
Anode 2 on the left and anode 1 on the right. The grid is a pair of vertical rods.
The filament is a single tungsten wire. The lower two thirds is still standing.
The wide glass tube envelope is 30 mm in diameter, and excluding the B4 base pins is 96 mm tall.
References: Patent, & 1047. Type Negatron was first introduced in 1919.